1. Field of Invention
The present invention relates to a lead frame structure. More particularly, the present invention relates to a lead frame structure capable of preventing semiconductor package body from warping.
2. Description of Related Art
In general, the manufacturing of integrated circuit (IC) can be divided into three separate stages: fabricating silicon wafers, forming integrated circuit on the silicon chips and packing the integrated circuit (IC) chips. Packaging of the IC chip is the final step in the production of IC products. In packaging an IC chip, the lead frame serves as a carrier for the silicon chip as well as an intermediate element linking the silicon chip with external printed circuit board (PCB).
FIG. 1 is a schematic, top view showing a typical lead frame structure. As shown in FIG. 1, the lead frame has a flat, sheet-like structure. Each lead frame can be divided into two sections, namely, a planar section and leads section. The planar section includes a silicon chip carrier commonly known as a bar pad or die pad 12. The lead section can be further subdivided into internal lead sections 14, lead shoulders 16 and external lead sections 18. The lead frame can also be divided into a package area 22 (the region enclosed by larger, dotted rectangle), also known as an IC packaging area and external leads region. The packaging area 22 further includes a bonding area 20 or coin area within. The bonding area 20 is an area where metallic wires are bonded. The internal lead sections 14 that are within the coin area 20 are often referred to as coin lead tips 24. Each coin lead tip 24 serves as a base for fixing a conductive wire extending from the silicon chip to the leads.
Furthermore, to facilitate automation of production, the lead frame further includes two side rails 26 for connecting with individual lead frames. Pilot holes 28 are also bored near the centerline of the side rails 26 for aligning the lead frame during assembly. Two tie bars 30 attached the die pad 12 to side rail 26. In addition, dam bars 32 connect all the lead shoulders 16 to the side rails 26. Besides holding all the leads in position while transfer molding is carried out, the dam bars 32 are used to prevent plastic compound from leaking out of the package.
Conventional method of packaging a semiconductor IC chip includes the steps of placing a silicon chip on the die pad 12, and then fixing the chip onto the die pad 12 with epoxy. Subsequently, the bonding pads of the silicon chip and the coin lead tips 24 of the leads 14 are electrically connected by attaching gold or aluminum wires in a bonding operation. Thereafter, the packaging area 22 is completely enclosed using an insulating material. Finally, the dam bars 32 are trimmed and the external leads are formed to obtain a complete IC package.
However, the original lead frame design can affect the quality of final IC package. For example, improper design of lead frame can lead to a high level of warping in the IC package body. FIGS. 2A and 2B are two, schematic, cross-sectional views showing two possible mode of warping for an IC package. The package body 40 in FIG. 2A warps up while the package body 44 in FIG. 2B warps down. Both types of warping will lead to immense difficulties in soldering their respective leads 42 and 46 onto a printed circuit board using conventional surface mounting technique (SMT).
In light of the foregoing, there is a need to provide an improved lead frame structure capable of preventing the warping of semiconductor package body.